Under the Cosh, this may as well have been called, which is perhaps a little unfair, but you didn't have to speed-read it inside a week. It's not that this is a bad book. It is, in many ways, a good book: King's take on the America of Bush and 9/11, a nation on the verge of environmental and moral collapse. But it is, in so many other ways, too much, too big, too long. And too Stephen King.

Even diehard fans of his peerless imagination, of whom there are justifiably many millions, will struggle with the sheer heft of the thing: it's like carrying around something which is simply wrongly weighted for a book, a hefty dead cormorant or some such, and after a little while it begins to feel like carrying around a grudge.

King writes short stories splendidly well and has won awards for them. He has a bizarre little idea and everyone goes: "Oh of course, why could I never think of that?" Normally, however, he judges it just so: the power of the idea is equalled by the length of the execution. Here he's got the proportions wrong.

It is a fine idea. A small, typical Maine community finds itself, one day in the very recent past, cut off from the rest of the world by an invisible, impenetrable dome, or Dome. Yes, I know The Simpsons Movie did the same thing, but King reportedly began this 25 years ago. It is not, this being King, a gentle awakening. The dome simply appears one second (miles high, as we soon learn, and extending way below bedrock), and when it appears some hands are pulling out root vegetables and are thus severed, while little planes crash and leave sinister smudges.

Some of the early goosebumps come when the eventual hero, Iraq vet Dale "Barbie" Barbara, and a new chum from the other side of the barrier (they can hear each other, and a little air can pass between them, but that's it) walk for miles in parallel, trying to find if it's a wall with an end or a… well, a dome.

From then on, we're along well-established lines, from Nevil Shute and before, inflected with the contemporary terror of environmental crisis: the air going bad, the water running out. The community goes to pot. A very bad fat man takes over, with guns. Religious zealots go (even more) mad. The mob, almost, rules, thwarted by a few good oddballs. There is comradeship, love, repentance.

There is also paranoia, blame and violations in the name of "security", and it's not hard to see the satire on Bush's America, especially when the main route on the "safe" side of Chester's Mill is the 119. These last few sentences seem terribly reductive; it's better than that. King reads widely, writes widely: there are glancing references to everything from Eliot to Melville to his fellow thriller writer Lee Childs. The existential explanation for the dome is beautifully managed, warmed up and hinted and, yes, keeps the pages turning.

The horror is also there. Partly, simply, through the language: King loves language and the way people use it. Take the terrible nastiness of redneck "Junior" Rennie, exposed when a girl's robe falls open and we hear his thoughts about her "breeding-farm", her "goddamn itchy breeding-farm that was all the fuckin trouble". His father, the real baddie, Big Jim, meanwhile, never swears but uses biblical euphemisms instead.

So much is sinister, so much plotted with grand intentions and lucid resolution. But despite the book's cover boast that it "took over 25 years to write", it turns out, in King's own honest words – for this is an honest and a brilliant and busy and moral man – at the very end, that it was written between 22 November 2007 and 14 March this year. He had a grand idea, a long time ago, then hammered it out recently in a year and a half. He could have done it as skilfully in a month and saved us the hernias.